OBJECTIVE To assess the effect of dietary restriction (DR) on energy expenditure (EE) in male and female rhesus monkeys. RESULTS At the 108 month (males) and 42 month (females) assessments, total energy expenditure (kJ/min) was lower in DR than control (C) animals. In the females, this difference was due entirely to a decrease in nighttime EE (p=0.023) in DR animals while daytime EE was not different between groups. EE was not different between DR and C animals when day or night values were expressed as a function of lbm or total mass. The increase in energy expenditure with mealtime was greater in DR than control females (1.11 vs 0.69 kJ/min; p=0.02). The ratio of day/night EE was also 25% greater in the DR compared to C animals. At 108 months of DR in the males, both day and night EE (kJ/min) tended to be greater in the C than DR monkeys. These differences disappeared when EE was expressed as a function of lbm or total mass. Mealtime EE or day/night EE were not different (p>0.10) between DR and C animals. DISCUSSION The mechanism responsible for lifespan extension with DR is unknown, however, a sustained decrease in energy expenditure has been proposed to play a central role in the action of this treatment. We have previously shown that DR results in an initial decrease in mass-adjusted energy expenditure which disappeared at later assessments. At the 108 (males) and 42 (females) month assessments, mass-adjusted energy expenditure was not different between groups. Dietary restriction, however, does appear to change the pattern of daily of EE. In both groups, nighttime or resting EE (kJ/min) showed the greatest difference between dietary restricted and control animals. The ratio of nighttime to daytime EE (an indicator of physical activity EE) tended to be higher in dietary restricted than control monkeys. These results suggest that DR decreases resting EE while maintaining or increasing activity-associated EE. Organ-specific and cellular energy expenditure measurements, however, will be required to firmly establish the effect of DR on EE. FUTURE DIRECTIONS We plan to continue to monitor changes in whole animal EE with DR. To better determine the effect of DR on EE, we plan use arterio-venous difference techniques to measure organ-specific energy expenditure. We also plan to continue current studies investigating the effects of aging and dietary restriction on mitochondrial energy metabolism. KEY WORDS dietary restriction, energy expenditure, aging FUNDING NIH PO1 AG11915